Remove duplicate HTTP status codes from Core::Error.

Consolidate the list of HTTP status codes into Core::HTTP. Add tests.

Add a helper to return the http status message for a given status code.
(Used in Core::Error in place of the dup'd list of error codes.)
Update Changelog.
Merge branch 'veryrusty/http_code_dup' into devel

* veryrusty/http_code_dup:
  Update Changelog.
  Remove duplicate HTTP status codes from Core::Error.